1/2 Day exploring around London=Grand Union Canal

The Grand Union runs from London to Birmingham. While the main line runs from the Thames in West London, the Paddington Arm and the Regents Canal runs across North London, and is a very pretty place to spend 1/2 a day.
Through connections into other waterways it actually runs right around north London, and out into the East London docklands (and the Thames again in East London) and on to Hertfordshire.
See this link for more details. As you can see it cuts right through London Zoo and more or less passes the British Museum. Lots of places to eat and the Camden Market.
Plenty of places to stay on that side, and very accessible into the London transport network for LHR.
Walks along the towing/tow path. Given a few weeks you can walk to Birmingham along the canal!!
A bit of country and history in London and something a bit different than the well known sights of central London. A place to unwind before the journey home.
https://canalrivertrust.org.uk/enjoy-the-waterways/canal-and-river-network/paddington-arm-grand-union-canal
https://en.wikipedia.org/wiki/Camden_Market.
Boat trips on the waterbus.
https://www.londonwaterbus.com/regents-canal-waterbus
If you prefer the sights then Windsor is easily accessible for Heathrow next day, but 1/2 day is very rushed to take in Windsor. That really wants more time.

You can see a section of the Grand Union Canal, with houseboats moored along its sides, by walking just outside London’s Paddington Station, where the Heathrow Express train brings people into London from Heathrow Airport. In March 2020, a week before the Pandemic was announced, we took the London Waterbus Co. cruise on the canal. Their Gardenia boat went from Little Venice, a triangular lagoon on the canal, through tunnels and past other boats and backyards of buildings, then sections of the London Zoo on both sides, and eventually let us off at the Camden Market. They sail back back to Little Venice, but we just did the one way. It took about an hour. There was supposed to be a guided narration during the trip, explaining points of interest along the way, but the alleged announcer spent 99% of his time sitting up at the front of the enclosed seating space, chatting with a small group of other passengers. I asked another member of the crew about the tunnels, and how boats got through before there were motorized boats (answer: they laid on their backs on top of the boat, and pushed against the ceiling of the low tunnel with hands and feet), but if I hadn’t asked, they seemingly wouldn’t have volunteered the information.

It was lunchtime, but the much-hyped Camden Market was basically cheap jewelry stands and not much in the way of food stalls, so we exited the market, and got lunch at a restaurant on Camden High Street, a couple of blocks away. We got back into central London by walking south through Regents Park, where the zoo is located.

Thank you both for your replies. We did the canal tour from Camden. It was a highlight for us to go along the route. I got some awesome pics as we floated under some of the bridges and could see the canal boats.

I want to stay somewhere that we can relax and enjoy some different scenery. I know that there is a portion of the Grand Union Canal close to Heathrow so we could try to stay nearby and not feel rushed. Just wasn't sure if it was worth a look. I haven't heard of it before... but we like doing things off the beaten track, if possible. Hidden gems.

I was just hoping to stay somewhere that we can easily catch the bus or tube to Heathrow. I believe it's the Piccadilly line into Terminal 2 & 3.

Thanks so much for your replies.

If you're really interested in canals, visit the London Canal Museum which is in an old refrigerated warehouse near King's Cross.